Leather tanning by the ancients was probably the first application of chemical science to every day life; even the chemistry of food preparation did not precede it. It is no surprise, therefore, that limitless methods for tanning leather have been developed to date. Generally, however, leather is produced by treating a hide or skin with one or more tanning agents, whereby the tanning process prevents degredation and putrefaction and also prevents the leather from drying into a stiff, boardy product. Many varied tanning agents are known in the art, and each different type provides certain advantages and yields leather having distinct characteristics.
Of the various tanning agents available, mineral tanning agents traditionally have been preferred, in lieu of vegetable-tanning agents, in the preparation of strong leathers having good volume and flexural strength. Of the mineral tanning agents, aqueous solutions of basic salts of chromium have been universally accepted and consistently preferred over other mineral tanning agents, such as the basic salts of zirconium and aluminum, because these "chrome-tanning" agents consistently yield superior products. Unfortunately, chrome-tanning agents also yield pollutant chrome-tanning effluents, which are undesirable for environmental reasons. The chromium containing effluents result from the disposal of shavings and leachings from dumped shavings and, of course, from the disposal of the chrome-tanning liquors.
Because chromium recovery or recycling of the spent chrome is not always as effective as expected, the leather industry has long needed a chromium-free tanning composition to eliminate the environmental difficulties associated with chrome-tanning agents. Furthermore, the rapidly rising costs of labor and heavy chemicals and the dependence of the United States on foreign sources of chromium ore also create a need for leather-tanning methods which yield products comparable to those tanned with chrome-tanning agents, without the use of chromium salts.
Certain chromium-free tanning agents are known in the art. These tanning agents, however, do not yield leather having a "chrome" character, but instead yield leathers which are alternatively softer, stretchier, weaker, thinner or more papery than chrome leathers.
The best known chromium-free tanning agents are the vegetable tans. Vegetable tans are aqueous extracts from plant leaves, barks, etc., to yield aqueous solutions or suspensions of large polyphenol molecules with some acidic groups and high secondary valency potentials (dipole hydrogen bonds). The acidic groups may combine with the basic groups of the hide protein to displace the water of hydration with a sheath of vegetable tan molecules. Upon drying of the hide, these vegetable tan molecules prevent shrinking and undue crosslinking of the hide and yield a preserved, but not stiff, leather product. Relatively high quantities of vegetable tans, as compared with mineral tans, must be incorporated into leather to give it firmness and flexural strength.
Many synthetic tanning agents, or syntans, are also well-known chromium-free tanning compositions. Although syntans may differ widely in actual chemical composition, syntans resemble vegetable tans in that they usually have a strong ionic charge and thus are strongly attracted to complementary ionic groups on the protein molecules in the hide. In the same general manner as the vegetable tans, therefore, the syntans dehydrate and preserve the leather without permitting it to shrink into a stiff, boardy product.
Tanning agents containing the basic salts of zirconium and aluminum function in a manner similar to chrome-tanning agents, but do so without chrome. Unfortunately, these zirconium and aluminum tanning agents cannot produce leather characteristic of the chrome-tanning process, but instead yield products which shrink more readily upon immersion in boiling water or during storage and which lack the smooth, flat, flexible grain and resilient feel of chrome-tanned leather. In addition, these tanning processes are frequently even more complicated, time-consuming and expensive than the chrome-tanning processes which they replace.
The various types of tanning agents have been combined, on occasion, to give "combination tanned" leathers. Neither the known chromium-free combination tanning processes nor the individual chromium-free tanning agents themselves have ever produced a leather product having the superior characteristics of chrome-tanned leather. Accordingly, the need persists for a chromium-free tanning process which, without the use of chromium salts, yields a leather demonstrating the various advantages characteristic of chrome-tanned hides and skins. More particularly, the need persists for a chromium-free tanning process suitable for tanning hides for use in strong, durable and weather-resistant footwear such as military shoes and boots.